The present invention relates to a system for transmitting the power of an engine to four wheels of a four-wheel drive vehicle.
In a conventional four-wheel drive vehicle having a central differential, which is called a full time four-wheel drive vehicle, the central differential operates to uniformly distribute the torque of an engine to front wheels and to rear wheels.
It is known that the distribution ratio of torque has influences on driveability, steerability, stability, starting characteristic and others. Namely, at high vehicle speed the distribution to the front wheel is increased so that stable cornering characteristic is established, and at low vehicle speed distribution to the rear wheels is increased so that steerability is improved.
Japanese Patent Laid Open No. 56-43031 discloses such a transmitting system, in which the distribution ratio of torque for front wheels to rear wheels is controlled depending on the vehicle speed detected by a vehicle speed sensor.
However, vehicle speed is not sufficient as a factor for controlling the torque distribution. For example, even if a rear trunk of a motor vehicle is loaded with a heavy cargo, the ratio of front and rear torques is the same as the ratio at light load. Accordingly, torque distribution becomes unbalanced, so that accelerating ability and up hill ability reduce. Further, the coefficient of friction of road surface affects the steerability and stability.